This invention relates to electronic rectifier circuits and, more particularly, to single phase circuits having a low input current distortion.
A basic single phase rectifier circuit includes a full wave bridge for rectifying an AC input voltage and a circuit branch including the series connection of an inductor and a capacitor which is connected across the output terminals of the bridge. The capacitor voltage is delivered to a load. Efficient conversion of single phase AC to DC voltage is hampered by non-sinusoidal input currents which result from the operation of such circuits. If the inductance of the inductor is very large, the input current of the bridge rectifier approximates a square wave. For more practical values of inductance, the input current consists of the sum of a square wave and a lagging fundamental component of the current. Such input currents have a very large total harmonic distortion, which causes the input power factor to be low, limiting the power available from a given wire or protective device. The distorted input currents will cause distortion in the line voltage. Ideally, the input current should look like the input voltage, so that the rectifier appears as a linear, resistive load to the rest of the power system, yielding unity input power factor and an ideal current crest factor of 1.414.
To reduce the input current distortion of the basic rectifier circuit described above, a boost converter can be added by placing a diode between the inductor and capacitor and connecting a switching device across the series connection of the diode and capacitor. The switching device can be turned on and off to develop a DC voltage on the capacitor which exceeds the peak of the AC input- voltage. With the switch off, the rectified output of the bridge is connected to the capacitor through the diode. The switch is turned on to increase current through the inductor and turned off to decrease current through the inductor. Turning off the switch dumps the energy from the inductor to the capacitor. For 60 Hz inputs, the switch may be operated at a rate of 40 kHz to 75 kHz, thereby providing precise control of the input current to match the input voltage waveform This is a switching rate of about 1000 times the input line frequency
For AC systems operating at high line frequencies, such as 20 kHz, this high frequency ratio is not practical. For high efficiency, a ratio of 10 times the line frequency would be more practical. A lower switching frequency requires the use of a larger inductor to keep the ripple current down to a reasonable level. The larger inductor causes a cross-over distortion problem. Near zero cross-over, there is no input voltage available to drive current into the inductor The current lags the input voltage causing a notch effect in the current waveform with low order harmonics which are difficult to filter.
It is therefore desirable to devise a single phase rectifier circuit which operates with relatively low input current distortion but operates at a relatively low switching rate such that it is practical for use in relatively higher frequency AC systems.